Modification of alkaline earth silicate fibres

ABSTRACT

A method of making refractory alkaline earth silicate fibres from a melt, including the use as an intended component of alkali metal to improve the mechanical properties of the fibre in comparison with a fibre free of alkali metal.

REFERENCE TO RELATED APPLICATION

The application is a divisional application and claims the benefit ofU.S. application Ser. No. 11/263,655 filed Oct. 31, 2005, which claimsthe benefit of priority from applicants' provisional application60/717,516 filed Sep. 15, 2005 now expired and British patentapplications GB 0424190.7 filed Nov. 1, 2004 and GB 0502701.6 filed Feb.9, 2005, all of which are relied on and incorporated herein byreference.

INTRODUCTION AND BACKGROUND

This invention relates to alkaline earth silicate fibres.

Inorganic fibrous materials are well known and widely used for manypurposes (e.g. as thermal or acoustic insulation in bulk, mat, orblanket form, as vacuum formed shapes, as vacuum formed boards andpapers, and as ropes, yarns or textiles; as a reinforcing fibre forbuilding materials; as a constituent of brake blocks for vehicles). Inmost of these applications the properties for which inorganic fibrousmaterials are used require resistance to heat, and often resistance toaggressive chemical environments.

Inorganic fibrous materials can be either glassy or crystalline.Asbestos is an inorganic fibrous material one form of which has beenstrongly implicated in respiratory disease.

It is still not clear what the causative mechanism is that relates someasbestos with disease but some researchers believe that the mechanism ismechanical and size related. Asbestos of a critical size can piercecells in the body and so, through long and repeated cell injury, have abad effect on health. Whether this mechanism is true or not regulatoryagencies have indicated a desire to categorise any inorganic fibreproduct that has a respiratory fraction as hazardous, regardless ofwhether there is any evidence to support such categorisation.Unfortunately for many of the applications for which inorganic fibresare used, there are no realistic substitutes.

Accordingly there is a demand for inorganic fibres that will pose aslittle risk as possible (if any) and for which there are objectivegrounds to believe them safe.

A line of study has proposed that if inorganic fibres were made thatwere sufficiently soluble in physiological fluids that their residencetime in the human body was short; then damage would not occur or atleast be minimised. As the risk of asbestos linked disease appears todepend very much on the length of exposure this idea appears reasonable.Asbestos is extremely insoluble.

As intercellular fluid is saline in nature the importance of fibresolubility in saline solution has long been recognised. If fibres aresoluble in physiological saline solution then, provided the dissolvedcomponents are not toxic, the fibres should be safer than fibres whichare not so soluble. Alkaline earth silicate fibres have been proposedfor use as saline soluble, non-metallic, amorphous, inorganic oxide,refractory fibrous materials. The invention particularly relates toglassy alkaline earth silicate fibres having silica as their principalconstituent.

International Patent Application No. WO87/05007 disclosed that fibrescomprising magnesia, silica, calcia and less than 10 wt % alumina aresoluble in saline solution. The solubilities of the fibres disclosedwere in terms of parts per million of silicon (extracted from the silicacontaining material of the fibre) present in a saline solution after 5hours of exposure. WO87/05007 stated that pure materials should be usedand gave an upper limit of 2 wt % in aggregate to the impurities thatcould be present. No mention of alkali metals was made in this patent.

International Patent Application No. WO89/12032 disclosed additionalfibres soluble in saline solution and discusses some of the constituentsthat may be present in such fibres. This disclosed the addition of Na₂Oin amounts ranging from 0.28 to 6.84 wt % but gave no indication thatthe presence of Na₂O had any effect.

European Patent Application No. 0399320 disclosed glass fibres having ahigh physiological solubility and having 10-20 mol % Na₂O and 0-5 mol %K₂O. Although these fibres were shown to be physiologically solubletheir maximum use temperature was not indicated.

Further patent specifications disclosing selection of fibres for theirsaline solubility include for example European 0412878 and 0459897,French 2662687 and 2662688, PCT WO86/04807, WO90/02713, WO92/09536,WO93/22251, WO94/15883, WO97/16386 and U.S. Pat. No. 5,250,488.

The refractoriness of the fibres disclosed in these various prior artdocuments varies considerably and for these alkaline earth silicatematerials the properties are critically dependent upon composition.

As a generality, it is relatively easy to produce alkaline earthsilicate fibres that perform well at low temperatures, since for lowtemperature use one can provide additives such as boron oxide to ensuregood fiberisation and vary the amounts of the components to suit desiredmaterial properties. However, as one seeks to raise the refractorinessof alkaline earth silicate fibres, one is forced to reduce the use ofadditives since in general (albeit with exceptions) the more componentsare present, the lower the refractoriness.

WO93/15028 disclosed fibres comprising CaO, MgO, SiO₂, and optionallyZrO₂ as principal constituents. Such fibres are frequently known as CMS(calcium magnesium silicate) or CMZS ((calcium magnesium zirconiumsilicate) fibres. WO93/15028 required that the compositions used shouldbe essentially free of alkali metal oxides. Amounts of up to 0.65 wt %were shown to be acceptable for materials suitable for use as insulationat 1000° C. WO93/15028 also required low levels of Al₂O₃ (<3.97%).

WO94/15883 disclosed a number of such fibres usable as refractoryinsulation at temperatures of up to 1260° C. or more. As withWO93/15028, this patent required that the alkali metal oxide contentshould be kept low, but indicated that some alkaline earth silicatefibres could tolerate higher levels of alkali metal oxide than others.However, levels of 0.3% and 0.4% by weight Na₂O were suspected ofcausing increased shrinkage in materials for use as insulation at 1260°C. The importance of keeping the level of alumina low was stressed isstressed in this document.

WO97/16386 disclosed fibres usable as refractory insulation attemperatures of up to 1260° C. or more. These fibres comprised MgO,SiO₂, and optionally ZrO₂ as principal constituents. These fibres arestated to require substantially no alkali metal oxides other than astrace impurities (present at levels of hundredths of a percent at mostcalculated as alkali metal oxide). The fibres have a general composition

SiO₂ 65-86% MgO 14-35%with the components MgO and SiO₂ comprising at least 82.5% by weight ofthe fibre, the balance being named constituents and viscosity modifiers.Such magnesium silicate fibres may comprise low quantities of otheralkaline earths. The importance of keeping the level of alumina low wasstressed is stressed in this document.

WO2003/059835 discloses certain calcium silicate fibres certain calciumsilicate compositions for which fibres show a low reactivity withaluminosilicate bricks, namely:

-   -   65%<SiO₂<86%    -   MgO<10%    -   14%<CaO<28%    -   Al₂O₃<2%    -   ZrO₂<3%    -   B₂O₃<5%    -   P₂O₅<5%    -   72%<SiO₂+ZrO₂+B₂O₃+5*P₂O₅

-   95%<SiO₂+CaO+MgO+Al₂O₃+ZrO₂+B₂O₃+P₂O₅

This patent also discloses the use of La₂O₃ or other lanthanideadditives to improve the strength of the fibres and blanket made fromthe fibres. This patent application does not mention alkali metal oxidelevels, but amounts in the region of ˜0.5 wt % were disclosed in fibresintended for use as insulation at up to 1260° C. or more.

WO2003/060016 claims a low shrinkage, high temperature resistantinorganic fiber having a use temperature up to at least 1330° C., whichmaintains mechanical integrity after exposure to the use temperature andwhich is non-durable in physiological fluids, comprising thefiberization product of greater than 71.25 to about 85 weight percentsilica, 0 to about 20 weight percent magnesia, about 5 to about 28.75weight percent calcia, and 0 to about 5 weight percent zirconia, andoptionally a viscosity modifier in an amount effective to render theproduct fiberizable.

EP 1323687 claims a biosoluble ceramic fiber composition for a hightemperature insulation material comprising 75-80 wt % of SiO₂, 13-25 wt% of CaO, 1-8 wt % of MgO, 0.5-3 wt % of ZrO₂ and 0-0.5 wt % of Al₂O₃,wherein (ZrO₂+Al₂O₃) is contained 0.5-3 wt % and (CaO+MgO) is contained15-26 wt %.

Alkaline earth silicate fibres have received a definition in theChemical Abstract Service Registry [Registry Number: 436083-99-7] of:

-   -   “Chemical substances manufactured in the form of fibers. This        category encompasses substances produced by blowing or spinning        a molten mixture of alkaline earth oxides, silica and other        minor/trace oxides. It melts around 1500° C. (2732° F.). It        consists predominantly of silica (50-82 wt %), calcia and        magnesia (18-43 wt %), alumina, titania and zirconia (<6 wt %),        and trace oxides.”

This definition reflects European Health and Safety regulations whichimpose special labelling requirements on silicate fibres containing lessthan 18% alkaline earth oxides.

However as is clearly indicated in relation to WO2003/059835,WO2003/060016 and EP 1323687, the silica content of alkaline earthsilicate fibres is increasing with the demand for higher usetemperatures and this is leading to lower alkaline earth contents.

The present invention is applicable not only to alkaline earth silicatefibres in this narrow definition reflected in the Chemical Abstractsdefinition, but also to alkaline earth silicate fibres having lowerlevels of alkaline earth oxides.

Accordingly, in the present specification alkaline earth silicate fibresshould be considered to be materials comprising predominantly of silicaand alkaline earth oxides and comprising less than 10 wt % alumina [asindicated in WO87/05007—which first introduced such fibres], preferablyin which alumina, zirconia and titania amount to less that 6 wt % [asindicated in the Chemical Abstracts definition]. For regulatory reasons,preferred materials contain more than 18% alkaline earth metal oxides.

The prior art shows that for refractory alkaline earth silicate fibres,alkali metals have been considered as impurities that can be toleratedat low levels but which have detrimental affects on refractoriness athigher levels.

SUMMARY OF THE INVENTION

The applicant has found that, contrary to received wisdom in the fieldof refractory alkaline earth silicate fibres, the addition of minorquantities of alkali metals within a certain narrow range improves themechanical quality of fibres produced (in particular fibre strength)without appreciably damaging the refractoriness of the fibres.

Accordingly, the present invention provides a method of makingrefractory alkaline earth silicate fibres from a melt, comprising theinclusion as an intended melt component of alkali metal to improve themechanical and/or thermal properties of the fibre in comparison with afibre free of alkali metal.

Preferably, the amount of alkali metal (M) expressed as the oxide M₂O isgreater than 0.2 mol % and preferably in the range 0.2 mol % to 2.5 mol%, more preferably 0.25 mol % to 2 mol %.

By “a fibre free of alkali metal” is meant a fibre in which all othercomponents are present in the same proportions but which lacks alkalimetal.

The alkali metal is preferably present in an amount sufficient toincrease the tensile strength of a blanket made using the fibre by >50%over the tensile strength of a blanket free of alkali metal, and lessthan an amount that will result in a shrinkage as measured by the methoddescribed below of greater than 3.5% in a vacuum cast preform of thefibre when exposed to 1250° C. for 24 hours.

It will be apparent that the alkali metal may be provided either as anadditive to the melt (preferably in the form of an oxide), or by usingas ingredients of the melt appropriate amounts of materials containingalkali metal as a component or impurity, or both as an additive and as acomponent or impurity. The invention lies in ensuring that the melt hasthe desired quantity of alkali metal to achieve the beneficial effectsof the invention.

The invention may be applied to all of the prior art alkaline earthsilicate compositions mentioned above.

BRIEF DESCRIPTION OF DRAWINGS

The scope and further features of the invention will become apparentfrom the claims in the light of the following illustrative descriptionand with reference to the drawings in which:

FIG. 1 is a graph showing tensile strength/density plotted against meltstream temperatures as determined in a production trial for a number offibres of differing Na₂O content;

FIG. 2 is a graph plotting maximum, average, and minimum values oftensile strength/density against Na₂O content for the same fibres;

FIG. 3 is a graph of experimentally determined temperature/viscositycurves for a range of compositions;

FIG. 4 is a graph showing shot content plotted against Na₂O content forthe fibres of FIG. 1

FIG. 5 is a graph of shot content against Na₂O content for a differentrange of alkaline earth silicate fibres

FIG. 6 is a graph of linear shrinkages for alkaline earth silicatefibres of varying composition, compared with known refractory ceramicfibre (RCF) fibres

FIG. 7 is a graph of the effect on blanket strength of sodium additionto a range of alkaline earth silicate fibres

FIG. 8 contrasts micrographs showing various fibres after exposure to arange of temperatures

FIG. 9 is a graph comparing measured thermal conductivities for a rangeof fibres.

DETAILED DESCRIPTION OF INVENTION

The inventors produced fibre blanket using a production trial line attheir factory in Bromborough, England. Fibre was produced by forming amelt and allowing the melt to fall onto a pair of spinners (as isconventionally known).

The base melt had a nominal composition in weight percent:

SiO₂ 73.5 CaO 25 La₂O₃ 1.5with other components forming minor impurities and sodium oxide beingadded in specified amounts.

The melt stream temperature was monitored using a two colour pyrometer.

Fibres produced from the spinners were passed onto a conveyer and thenneedled to form blanket in a conventional manner.

The blanket thickness, density, and tensile strength were measured forfibres produced using a range of conditions.

The blanket was produced with a view to determining the effect on fibrequality of melt stream temperature, since it was believed that this hadan effect on fibre quality.

The inventors also decided to add alkali metal oxides with the view offlattening the viscosity-temperature curve of the melt as this wasthought a relevant factor in fibre production as explained furtherbelow.

The results of these tests are set out in Table 1 and illustratedgraphically in FIGS. 1 and 2. In Table 1, the melt stream temperature,blanket thickness, blanket density, tensile strength and tensilestrength divided by density is shown for all compositions. [The tensilestrength divided by density is calculated to counteract the variationattributable to different amounts of material being in the blanket].Also for selected compositions the shrinkage of a preform at 1150° C.and 1250° C. was measured in the same manner as in WO2003/059835.

The first thing that is noteworthy is that the blanket strengths show ahigh variability. This is because the manufacture of a blanket involvesmany variables, including:

-   -   Composition of the melt    -   Temperature of the melt    -   Melt stream temperature    -   Shot content (melt that has solidified in the form of droplets        rather than fibres)    -   Fibre diameter    -   Fibre length    -   Needling conditions    -   Post-solidification thermal history

By producing a range of fibres on a single line and significantlyvarying only melt stream temperature and composition (each of which willhave an affect on shot content, fibre diameter and fibre length) it washoped to reduce such variability. However because a blanket is anaggregated body of individual fibres, there is inevitably a statisticalvariation in such aggregate properties as tensile strength.

As can be seen from FIG. 1 there appears to be relatively littlevariation in strength with melt stream temperature, but since the rangeof melt stream temperatures chosen was selected to encompass rangespreviously found to be effective, this is not surprising.

However, it can be seen that with progressive increases in Na₂O content,the strength tends to increase. FIG. 2 shows the maximum, minimum, andaverage strengths found for a range of compositions and it can be seenthat blanket strength shows a strong positive correlation with Na₂Ocontent. In contrast, the shrinkage of the fibres seemed barelyaffected.

The fibres with nominal zero Na₂O content of course had minor traceamounts (average measured content 0.038%—maximum 0.11%). Extrapolatingback to zero Na₂O gives an average tensile strength/density of 0.0675kPa/[kg/m³]. The average tensile strength/density for the addition of0.3% Na₂O is 0.1426. The increase in blanket strength is over 100% andsmaller additions (e.g. 0.25 mol %) would be expected to exceed a 50%improvement.

TABLE 1 % linear % linear Tensile Strength Melt Stream Blanket Blanketshrinkage shrinkage kPa (average of Tensile Temperature thicknessdensity 1150° C./24 1250° C./24 three strength/ ° C. (mm) (kg/m³) hourshours measurements) density Zero nominal Na₂O content 1750 7.20 118 5.670.048023 1750 8.18 109 8.33 0.076453 1750 16.87 161 15.33 0.095238 175015.12 169 15.67 0.092702 1750 15.71 134 16.00 0.119403 1750 20.51 14113.67 0.096927 1750 19.14 138 11.33 0.082126 1750 18.58 125 9.670.077333 1750 18.87 141 12.00 0.085106 1750 25.92 130 14.00 0.1076921750 24.49 140 17.00 0.121429 1750 15.88 166 13.47 0.081124 1750 17.34144 7.33 0.050926 1750 11.00 174 16.20 0.093103 1750 22.01 124 0.52 0.887.91 0.06379 1760 16.60 133 18.47 0.138847 1800 8.06 129 8.67 0.0671831800 22.04 132 12.92 0.097904 1800 21.97 139 13.62 0.09801 1850 7.75 1209.33 0.077778 1850 18.49 133 9.31 0.069962 1850 18.12 128 8.56 0.0669011850 17.19 123 5.33 0.043333 1850 24.49 125 5.26 0.042107 1900 21.83 11410.57 0.092708 1910 8.50 127 12.33 0.097113 1950 8.14 115 9.33 0.0811591950 8.92 115 10.00 0.086957 1990 19.39 123 10.67 0.086764 0.3 wt %nominal Na₂O content 1800 22.82 107 19.83 0.185327 1850 17.10 149 16.910.113512 1900 24.40 137 17.66 0.128881 0.5 wt % nominal Na₂O content1795 20.32 169 0.43 1.70 48.64 0.287811 1800 19.98 147 24.81 0.1689131800 25.25 136 16.17 0.118922 1800 18.64 153 34.24 0.223769 1800 18.02190 42.65 0.224456 1800 24.22 175 37.26 0.212895 1800 22.47 165 36.830.223212 1835 14.54 150 42.01 0.280067 1850 23.50 164 0.31 1.04 27.290.166789 1850 25.15 162 27.85 0.171681 0.7 wt % nominal Na₂O content1800 21.91 166 47.12 0.283835 1800 21.25 166 38.32 0.230863 1800 18.44161 53.64 0.333188 1800 19.22 163 38.74 0.237669 1800 19.95 144 0.481.11 33.35 0.231597 1850 26.04 175 0.48 0.90 38.41 0.219467 1850 23.48166 54.11 0.325984 1850 27.73 165 37.03 0.224404 1900 29.30 166 41.690.251165 1900 21.16 135 44.09 0.326617 1900 19.49 135 40.93 0.30316 195025.88 151 39.12 0.259073

Encouraged by this, and with a view to determining the upper limit ofalkali metal oxide that was appropriate, the inventors produced a rangeof further alkaline earth silicate fibres using an experimental rig inwhich a melt was formed of appropriate composition, tapped through a8-16 mm orifice, and blown to produce fibre in a known manner. (The sizeof the tap hole was varied to cater for the viscosity of the melt—thisis an adjustment that must be determined experimentally according to theapparatus and composition used). Shrinkage of preforms of the fibre at1150° C. and 1250° C. were measured in the same manner as inWO2003/059835. Total solubility in ppm of the major glass componentsafter a 24 hour static test in a physiological saline solution were alsomeasured for some of the examples.

The results of these studies are shown in Table 2. The fibres in theleft of the table were aimed at assessing the effect of addingapproximately equimolar amounts of alkali metal addition to calciumsilicate fibre containing La₂O₃ (as in WO2003/059835), whereas those tothe right were aimed at assessing the effect of varying the quantity ofNa₂O in such a fibre. While not conclusive, the results indicate thatfor these fibres Na₂O and K₂O show shrinkages no worse or even betterthan fibre free of Na₂O, whereas Li₂O appears detrimental to shrinkage.

However, this latter conclusion is thought unsafe since it wasdetermined that the lithium had been added in the form of lithiumtetraborate, and the boron addition may have had a significant effect.Until proven otherwise, the applicants are assuming that all alkalimetals can be used in the invention, but that the absolute amount ofalkali metal may vary from metal to metal and fibre to fibre. Thesolubility figures show that total solubility is slightly increased bythe addition of alkali metal oxide.

TABLE 2 Sample PAT PAT PAT Li₂O PAT K₂O BG-X-04- BG-X-04- BG-X-04- STD01 Na₂O 02 03 04 0305 0277 0279 Component Na₂O 0.26 0.95 0.12 0.24 0.60.72 1.14 MgO 0.38 0.39 0.36 0.36 0.35 0.38 0.36 Al₂O₃ 0.6 0.64 0.560.62 0.38 0.02 0 SiO₂ 72.58 72.47 72.43 72.40 73.26 73.58 73.76 K₂O 0.080.08 0.07 1.05 0.07 0.08 0.08 CaO 24.05 23.27 23.62 22.67 22.82 23.5223.22 TiO₂ 0.1 0.10 0.11 0.15 0.1 0.1 0.1 Fe₂O₃ 0.16 0.19 0.21 0.23 0.160.18 0.18 La₂O₃ (estimated) 1.3 1.3 1.3 1.3 1.3 1.3 1.3 Li₂O 0.34* %Linear Shrinkage  850° C./24 hours 0.38 0.21 0.22 1150° C./24 hours 1.050.88 1.58 0.63 0.47 0.36 0.59 1250° C./24 hours 1.08 1.08 1.71 0.79 0.480.69 0.84 % Thickness shrinkage 850° C./24 hours 0.42 0.71 1.31 1150°C./24 hours 0.93 0.71 1.44 1250° C./24 hours 0.91 0.72 6.43 StaticSolubility 24 hrs (ppm) 191 202 200 N/A

The right side of Table 2 shows firstly that only a ˜1% higher silicacontent has a big effect on shrinkage, giving a much lower shrinkage.For these fibres, linear shrinkage at 850° C./24hrs seemed unaffected byall soda additions tested, however the same is not true for thicknessshrinkage, although it is still low. At 1150° C./24hrs there is a slightincrease in both linear and through thickness shrinkage, but at 1250°C./24hrs through thickness whilst still acceptable grows moresignificantly for the highest soda addition. All of these figures areacceptable for some applications whereas other applications could nottolerate the highest Na₂O level tested.

The improvement in shrinkage with higher silica levels led the inventorsto look to materials containing still higher silica levels and theresults are set out in Table 3 below.

TABLE 3 Sample PAT Na₂O PAT Na₂O PAT Na₂O PAT Na₂O PAT Na₂O PAT Na₂O 0506 07 08 09 10 Component Na₂O 0.5 0.5 0.5 0.5 0.5 1.1 MgO 0.4 0.3 0.30.4 0.3 0.4 Al₂O₃ 0.6 0.5 0.6 0.8 0.6 0.8 SiO₂ 73.9 74.3 74.5 75.2 76.377.7 K₂O 0.1 0.1 0.1 0.1 0.1 0.1 CaO 23.6 22.9 22.6 22.0 21.4 19.3 TiO₂0.1 0.1 0.1 0.1 0.1 0.1 Fe₂O₃ 0.2 0.2 0.2 0.2 0.2 0.2 La₂O₃ 1.3 1.3 1.31.3 1.3 1.3 % Linear Shrinkage 1150° C./24 hrs 0.54 0.8 0.61 0.56 0.650.58 1250° C./24 hrs 1.1 1.07 N/A 0.84 0.86 N/A Static Solubility 24 hrs(ppm) 199 208 165 194 245 107

These results show low shrinkage and a reasonably high solubility acrossthe range. It appears that addition of alkali metal oxide may increasethe amount of silica that can be added to produce a workable alkalineearth silicate fibre, and perhaps with an acceptable solubility. This isof great significance since, in general, increasing silica contentpermits higher use temperatures for alkaline earth silicate fibres.

FIG. 6 shows the shrinkage at various temperatures of preforms of arange of alkaline earth silicate fibres. The reference SW613 refers tolanthanum containing materials of composition similar to those set outin Table 3 with varying silica contents as indicated but absent anyalkali metal addition. [Silica and calcia comprising most of thematerial with lanthanum oxide being present in about 1.3%]. One of thesefibres also has an addition of 2 wt % MgO. Also shown are shrinkages fora conventional aluminosilicate fibre (RCF) and a magnesium silicatefibre (MgO Silicate).

It can be seen that all of the SW613 fibres have a shrinkage lower thanthat of RCF and the MgO silicate fibres up to 1350° C. but risethereafter. However, there is a progressive increase in refractorinesswith increasing silica content. For the SW613 fibre containing 77 and79% SiO₂, the shrinkage remains below that of RCF and the MgO silicatefibres up to 1400° C. and better could be expected for higher silicacontents. In contrast, it can be seen also that addition of 2% MgO tothe SW613 compositions is detrimental to shrinkage. High silica alkalineearth silicate fibres are difficult to make and addition of alkalimetals to such compositions should improve the quality of such fibresand ease manufacture.

Having shown such effects the applicants conducted a trial to makeblanket on a production line, to see whether the initial results onshrinkage were confirmed. A base composition comprising:

SiO₂ 72.5-74 wt % CaO 24-26.5 wt % MgO 0.4-0.8 wt % Al₂O₂ <0.3 wt %La₂O₃ 1.2-1.5 wt %was used and varying amounts of Na₂O were added. Blanket having adensity 128 kg/m³ was produced having a thickness of ˜25 mm. Theresults, summarised in FIG. 7, show a dramatic increase in blanketstrength with Na₂O addition.

These findings relate to compositions containing La₂O₃ as a component,but similar effects of alkali metal additions are found with alkalineearth silicate fibres not containing La₂O₃ as a component.

The inventors also tested other alkaline earth silicate fibrescomprising predominantly magnesium as the alkaline earth component(magnesium silicate fibres) and the results are set out in Table 4.

This table shows that whereas Na₂O and K₂O have a small or largerespectively detrimental effect on shrinkage, Li₂O has hardly any effecton shrinkage. This does not imply no effect at all, the inventorsobserved that whereas the fibres with Na₂O and K₂O were similar tofibres without such additives (coarse) the fibre with Li₂O addition wassignificantly finer and of better quality. At lower quantities, Na₂O andK₂O may still give shrinkages that are tolerable in most applications.

TABLE 4 Sample 04 MgO 01 04 MgO 02 04 MgO 03 04 MgO 04 Component Na₂O0.0 0.5 0.0 0.0 MgO 20.0 19.1 19.6 18.3 Al₂O₃ 1.7 2.0 1.8 1.7 SiO₂ 77.677.5 77.8 78.2 K₂O 0.0 0.0 0.0 1.0 CaO 0.5 0.5 0.6 0.5 TiO₂ 0.1 0.1 0.10.1 Fe₂O₃ 0.5 0.5 0.5 0.5 Li₂O 0.3 % Linear Shrinkage 1150° C./ 2.533.53 2.34 5.59 24 hrs 1250° C./ 2.16 3.57 2.3 9.94 24 hrs StaticSolubility 24 hrs (ppm) 297 N/A 331 N/A

The purpose of adding alkali metal is to try to alter the viscositytemperature curve for alkaline earth silicates so as to provide a moreuseful working range for the silicates. FIG. 3 shows a graphexperimental viscosity/temperature curves for:

-   -   a high soda glass having the approximate composition in wt %:

SiO₂ 68 Na₂O 13.4 CaO 7.94 B₂O₃ 4.74 MgO 2.8 Al₂O₃ 2.66 Fe₂O₃ 1.17 TiO₂0.09 ZrO₂ 0.08 Cr₂O₃ 0.06

-   -   an alkaline earth silicate melt comprising the approximate        composition:    -   CaO 29    -   MgO 6%    -   SiO₂ 64.5

-   + others to 100%    -   and the same alkaline earth silicate melt comprising        respectively 1 wt % Na₂O and 2 wt % Na₂O as an additive.

The viscosity/temperature graph of the high soda glass is a smooth linerising as temperature falls.

For the known alkaline earth silicate melt (SW) the viscosity is lowerand then rises steeply at a critical temperature value (this is shown asa slope in the graph but that is an artefact of the graphing process—itactually represents a much steeper change).

Addition of Na₂O to the melt moves this rise in viscosity to lowertemperatures.

This extends the working range of the melt so that it becomes lessdependent upon temperature so increasing the tolerance of the melt tofibre forming conditions. Although the melt stream temperature isimportant, the melt cools rapidly during the fibre forming process andso a longer range of workability for the composition improves fibreformation. The addition of the alkali metal oxides may also serve tostabilise the melt stream so that for a given set of conditions there isan amount that reduces shot.

Additionally, it is surmised that in small quantities the alkali metaloxides serve to suppress phase separation in alkaline earth silicatefibres.

Since the alkaline earth silicate systems have a two liquid region intheir phase diagrams, the applicants suspect that addition of alkalimetal oxides may move the melts out of a two-liquid region into a singlephase region.

The addition also has the effect of lowering melt stream temperaturewhich may assist in stability.

The effectiveness of these measures is also shown by the amount of shotpresent in the finished material. In the fibre forming process, dropletsof melt are rapidly accelerated (by being flung off a spinning wheel orbeing blasted by a jet of gas) and form long tails which become thefibres.

However that part of the droplets that does not form fibre remains inthe finished material in the form of particles known in the industry as“shot”. Shot is generally detrimental to the thermal properties ofinsulation formed from the fibres, and so it is a general aim in theindustry to reduce the quantity of shot.

The applicants have found that addition of minor amounts of alkali metalto the melt has the effect of reducing the amount of shot, and this isshown in FIG. 4 for the lanthanum containing materials of Table 1, whereit can be seen that the shot content was reduced from ˜51% to ˜48%.

Similar effects apply to lanthanum free materials. Table 5 shows theanalysed compositions of a range of alkaline earth silicate fibres(having a lower maximum use temperature) made in accordance with thecompositions of WO93/15028, which were made by spinning using a meltstream temperature of 1380-1420° C., and with a pair of rotatingspinners.

FIG. 5 shows experimentally determined shot contents with error barsindicating one standard deviation about mean. It can be seen that in therange 0.35 to 1.5 wt % Na₂O, there is a statistical improvement in theshot content as a result of the addition. In particular, a 3% reductionin shot for a 0.35 wt % soda content is significant.

Since there seems no detrimental effect on shrinkage at such levels (andindeed a slight improvement) it can be seen that addition of alkalimetal oxides is beneficial for the production of such materials.

TABLE 5 Sample 04-C43-1 04C56-7 04C46-5 04C47-2 04C51-6 04C50-8 04C49-6Component Na₂O 0.11 0.35 0.66 1.01 1.47 2.03 2.46 MgO 4.78 5.90 5.185.47 5.71 5.76 6.20 Al₂O₃ 1.07 0.40 0.35 0.27 0.30 0.36 0.30 SiO₂ 65.165.16 65.07 64.96 65.91 66.15 65.24 P₂O₅ 0 0.00 0.00 0.00 0.00 0.00 0.00K₂O 0.08 0.08 0.08 0.07 0.07 0.07 0.07 CaO 28.92 27.84 28.47 28.12 26.2525.36 24.79 TiO₂ 0.02 0.02 0.03 0.01 0.02 0.03 0.02 Cr₂O₃ 0.02 0.02 0.020.02 0.02 0.02 0.02 Mn₃O₄ 0.03 0.03 0.03 0.03 0.03 0.03 0.03 Fe₂O₃ 0.20.19 0.19 0.18 0.18 0.18 0.18 ZnO 0 0.00 0.00 0.00 0.00 0.01 0.00 SrO0.01 0.01 0.01 0.01 0.01 0.01 0.01 ZrO₂ 0 0.00 0.00 0.00 0.00 0.00 0.00BaO 0 0.00 0.00 0.00 0.00 0.00 0.00 HfO₂ 0 0.00 0.00 0.00 0.00 0.00 0.00PbO n/a n/a n/a n/a n/a n/a n/a SnO₂ n/a n/a n/a n/a n/a n/a n/a CuO n/an/a n/a n/a n/a n/a n/a % Linear Shrinkage 1000° C./24 hours 1.42 1.331.54 4.18 1100° C./24 hours 1.39 1.20 1.77 4.85

Addition of the alkali metal should be at levels that do not excessivelydetrimentally affect other properties of the fibre (e.g. shrinkage), butfor different applications what is “excessive” will vary.

The fibres can be used in thermal insulation and may form either aconstituent of the insulation (e.g. with other fibres and/or fillersand/or binders) or may form the whole of the insulation. The fibres maybe formed into blanket form insulation.

Although initial work was primarily related to the addition of Na₂O toalkaline earth silicate fibres, the applicants discovered that when Na₂Owas used as the additive to high calcium-low magnesium fibres it had atendency to promote crystallisation (and hence powderiness of thefibres) after exposure to temperatures of ˜1000° C. This can be seen inFIG. 8 in which fibre a) -e) had base compositions falling in theregion:

SiO₂ 72-75 wt % CaO 22-26.5 wt % MgO 0.4-1 wt % Al₂O₂ <0.3 wt % La₂O₃1.2-1.5 wt %

Fibres a), b) and c) show the effect on surface appearance of fibresafter exposure to 1050° C. for 24 hours on fibres containing increasingamounts of Na₂O (from ˜0 through 0.5 wt % to 1.06 wt % respectively). Ascan be seen, the fibre absent Na₂O has a smooth appearance indicatinglittle crystallisation, whereas increasing Na₂O leads to an increase insurface roughness indicative of crystallisation.

In contrast, fibres d) and e) show that at 1100° C. a fibre containing˜0.5 wt % K2O is little different from a fibre free of K₂O, and onlystarts to show slight surface roughness at 1150° C.

Table 6 shows relative thermal conductivities of blankets havingapproximate density of 96 kg.m⁻³ formed from fibres having the principalingredients shown. It also shows thermal conductivities of theseblankets and these figures are shown in FIG. 9. It can be seen thataddition of Na₂O and K₂O seems to result in lower thermal conductivityfrom the blankets so showing improved insulating ability.

TABLE 6 Ca Silicate Ca Silicate Ca Silicate Mg Silicate with K₂O withNa₂O blanket Blanket addition addition Na₂O 0.22 0 0 1.06 MgO 0.4 19.130.74 0.96 Al₂O₃ 0.79 1.58 0.15 0.13 SiO₂ 73.94 79.08 74.7 72.1 K₂O 0.060 0.75 0 CaO 22.69 0.25 22.3 24.5 TiO₂ 0 0.06 0 0 Fe₂O₃ 0.16 0.38 0.04 0La₂O₃ 2.07 NA 1.36 1.26 Temperature° C. Thermal Conductivity (w · m⁻¹ ·K⁻¹)  200 0.06 0.06  400 0.12 0.11  600 0.35 0.35 0.21 0.2  800 0.590.57 0.33 0.34 1000 0.9 0.85 0.49 0.52 1200 1.3 1.2 0.67 0.75

The applicants have therefore identified further advantages of the useof alkali metal oxides as additives to alkaline earth silicate blanketmaterials, and particular advantage to the use of potassium. Inparticular, to avoid promotion of crystallisation by sodium, preferablyat least 75 mol % of the alkali metal is potassium. More preferably atleast 90%, still more preferably at least 95% and yet still morepreferably at least 99% of the alkali metal is potassium.

To test the mutual interaction of La₂O₃ and K₂O on the fibre propertiesa range of fibres were made into blankets and tested for shrinkage atvarious temperatures [24 hours at temperature].

It was found that La₂O₃ could be reduced and replaced by K₂O withoutsignificant harm to the shrinkage properties of the materials, but thisled to onset of crystallisation at lower temperatures than for the La₂O₃containing materials. However, replacement of La₂O₃ in part by aluminacured this problem. Table 7 indicates a range of materials tested, thetemperature at which crystallisation commenced, and temperature at whichthe crystals reached ˜1 μm in size. The materials all had a basecomposition of approximately 73.1-74.4 wt % SiO₂ and 24.6-25.3 wt % CaOwith all other ingredients amounting to less than 3% in total.

Crystals Crystallisation Coarsen Composition Starts @ ° C. ~1 mm @ ° C.CaO—SiO2—La2O3 (1.3%) 1100 1200 CaO—SiO2—K₂O (0.75%) 1000 1100CaO—SiO2—K₂O (0.75%)—La₂O₃ 1050 1150 (1.3%) CaO—SiO2—K₂O (0.75%)—La₂O₃1050 1150 (1.3%) CaO—SiO2—K₂O (0.8%)—La₂O₃ 1050 1200 (0.4%) CaO—SiO2—K₂O(0.6%)—La₂O₃ 1100 1200 (0.15%)—Al₂O₃ (0.94%)

Accordingly, a preferred range of compositions comprises:

-   -   72%<SiO₂<79%    -   MgO<10%    -   13.8%<CaO<27.8%    -   Al₂O₃<2%    -   ZrO₂<3%    -   B₂O₃<5%    -   P₂O₅<5%    -   95%<SiO₂+CaO+MgO+Al₂O₃+ZrO₂+B₂O₃+P₂O₅    -   M₂O>0.2% and <1.5%        in which M is alkali metal of which at least 90 mol % is        potassium.

More preferably SiO₂ plus CaO>95%, and usefully a preferred range ofcompositions comprises:

-   -   72%<SiO₂<75%    -   MgO<2.5%    -   24%<CaO<26%    -   0.5%<Al₂O₃<1.5%    -   ZrO₂<1%    -   B₂O₃<1%    -   P₂O₅<1%    -   M₂O>0.2% and <1.5%        in which M is alkali metal of which at least 90 mol % is        potassium.

A particularly preferred range is

-   -   SiO₂ 74±2%    -   MgO<1%    -   CaO 25±2%    -   K₂ O 1±0.5%    -   Al₂O₃<1.5%    -   98% <SiO₂+CaO+MgO+Al₂O₃+K₂O

And these preferred ranges may comprise additionally R₂O₃<0.5 wt % whereR is selected from the group Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb, Lu, Y or mixtures thereof.

During further trials a second range of fibres was found that gave goodresults. These fibres had the composition:

-   -   SiO₂=67.8-70%    -   CaO=27.2-29%    -   MgO=1-1.8%    -   Al₂O₃=<0.25%    -   La₂O₃=0.81-1.08%    -   K₂O=0.47-0.63%

These fibres had a high strength (80-105 kPa for a blanket of thickness˜25 mm and density ˜128 kg.m³) and and low shot content (˜41% totalshot).

The fibres may also be used in other applications where alkaline earthsilicate fibres are currently employed (e.g. as constituents of frictionmaterials).

1. A method of making refractory alkaline earth silicate fibrescomprising less than 10 wt % alumina from a melt, comprising theinclusion as an intended melt component of alkali metal to improve themechanical and/or thermal properties of the fibre.
 2. A method, asclaimed in claim 1, in which the amount of alkali metal (M) expressed asthe oxide M₂O, is in the range 0.2 mol % to 2.5 mol %.
 3. A method, asclaimed in claim 1, in which the alkali metal is included in an amountsufficient to increase the tensile strength of a blanket made using thefibre by >50% over the tensile strength of a blanket free of alkalimetal, and less than an amount that will result in an excessiveshrinkage at the intended maximum use temperature.
 4. A method, asclaimed in claim 1, in which the composition of the alkaline earthsilicate fibre and the alkali metal content is such that the shrinkage,as measured by the method of the description, of a vacuum cast preformof the fibres when exposed to 850° C. for 24 hours is no greater than3.5%.
 5. A method, as claimed in claim 4, in which the composition ofthe alkaline earth silicate fibre and the alkali metal content is suchthat the shrinkage, as measured by the method of the description, of avacuum cast preform of the fibres when exposed to 1000° C. for 24 hoursis no greater than 3.5%.
 6. A method, as claimed as claimed in claim 5,in which the composition of the alkaline earth silicate fibre and thealkali metal content is such that the shrinkage, as measured by themethod of the description, of a vacuum cast preform of the fibres whenexposed to 1150° C. for 24 hours is no greater than 3.5%.
 7. A method,as claimed as claimed in claim 6, in which the composition of thealkaline earth silicate fibre and the alkali metal content is such thatthe shrinkage, as measured by the method of the description, of a vacuumcast preform of the fibres when exposed to 1250° C. for 24 hours is nogreater than 3.5%.
 8. A method, as claimed as claimed in claim 1, inwhich the composition of the alkaline earth silicate fibre and thealkali metal content is such that the shrinkage, as measured by themethod of the description, of a vacuum cast preform of the fibres whenexposed to 1150° C. for 24 hours is no greater than 2 times theshrinkage of a fibre of the composition free of alkali metal.
 9. Amethod, as claimed in claim 8, in which the composition of the alkalineearth silicate fibre and the alkali metal content is such that theshrinkage, as measured by the method of the description, of a vacuumcast preform of the fibres when exposed to 1150° C. for 24 hours is nogreater than 1.2 times the shrinkage of a fibre of the composition freeof alkali metal.
 10. A method, as claimed in claim 6, in which thecomposition of the alkaline earth silicate fibre and the alkali metalcontent is such that the shrinkage, as measured by the method of thedescription, of a vacuum cast preform of the fibres when exposed to1400° C. for 24 hours is no greater than 3.5%.
 11. A method, as claimedin claim 1, in which the inclusion as an intended melt component of thealkali metal results in a reduction in shot content.
 12. A method, asclaimed in claim 1, in which the alkali metal (M) is present in anamount expressed as the oxide M₂O less than 2 mol %.
 13. A method, asclaimed in claim 12, in which the alkali metal is present in an amountless than 1.5 mol %.
 14. A method, as claimed in claim 13, in which thealkali metal is present in an amount less than 1 mol %.
 15. A method, asclaimed in claim 14, in which the alkali metal is present in an amountless than 0.75 mol %.
 16. A method, as claimed in claim 12, in which thealkali metal is present in an amount greater than or equal to 0.3 mol %.17. A method, as claimed in claim 16, in which the alkali metal ispresent in an amount greater than or equal to 0.4 mol %.
 18. A method,as claimed in claim 17, in which the alkali metal is present in anamount greater than or equal to 0.5 mol %.
 19. A method, as claimed inclaim 18, in which the alkali metal is present in an amount greater thanor equal to 0.6 mol %.
 20. A method, as claimed in claim 1, in which thealkaline earth silicate fibre comprises <10 wt % MgO, and in which thealkali metal M comprises predominantly sodium, potassium, or a mixturethereof.
 21. A method, as claimed in claim 20, in which at least 75 mol% of the alkali metal is potassium.
 22. A method, as claimed in claim21, in which at least 90 mol % of the alkali metal is potassium.
 23. Amethod, as claimed in claim 21, in which at least 95 mol % of the alkalimetal is potassium.
 24. A method, as claimed in claim 21, in which atleast 99 mol % of the alkali metal is potassium.
 25. A method, asclaimed in claim 1, in which the alkaline earth silicate fibrecomprises >15 wt % MgO, and in which the alkali metal M comprisespredominantly lithium.